1. Field of the Invention
The present invention relates to methods for characterizing physical properties of underground formations by transmitting sonic waves in the borehole and by processing resulting sonic waveform measurements and, in particular, to methods for selecting the acoustic frequency of transmitted sonic waves to optimally excite dipole flexural mode sonic waves in the borehole.
2. Description of the Related Art
In the development of natural hydrocarbon (e.g. oil) reservoirs, a borehole is typically drilled into the ground from a surface location. Downhole measurements of various phenomenon and properties are often made to determine various characteristics related to the underground resources or formations, or otherwise related to the drilling process. To make such measurements, various borehole sensors or detectors may be located in the drill bit, in the bottom hole (or borehole) assembly (BHA), in the drill string above the mud motor, or in any other part of the sub-surface drill string. Borehole sensors are often contained on a special tool, such as a wireline tool, which is lowered into the hole on a wireline cable. The downhole measuring tool may also contain various signal sources for generating signals in the borehole for detection by sensors in the tool after passing through the borehole and/or underground formations around the borehole.
The sensed or measured data is typically transmitted to the surface, where it can be stored, processed, or otherwise used, e.g. to monitor and control the drilling process. Data measured or sensed by the downhole tool is typically transmitted or telemetered back to receivers and processing equipment at the surface by various telemetry techniques and systems, such as by hard wired cables or wireline tools which contain electrical and/or fiber optic conductors which transmit data to the surface based on inductive coupling or other principles.
Telemetry techniques other than wireline telemetry are also sometimes used to transmit sensed data to the surface. MWD (measurements-while-drilling) and LWD (logging-while-drilling) techniques, for example, which are sometimes used for making downhole measurements, typically employ drilling fluid or mud pulse telemetry, electromagnetic telemetry, or acoustic telemetry through the drill string itself, to transmit sensed data to the surface. Acoustic borehole telemetry and related modulation schemes are described by S. P. Monroe, xe2x80x9cApplying Digital Data-Encoding Techniques to Mud Pulse Telemetry,xe2x80x9d Proceedings of the 5th-SPE Petroleum Computer Conference, Denver, Jun. 25th-28th, 1990, SPE 20236, pp. 7-16.
It is possible to determine properties of underground formations using measurements of acoustic/sonic waves that have passed through the formations. Thus, one type of measurement made downhole is measurement of sonic waves generated by a sonic generator or transmitter, which sonic waves have passed through the borehole and/or underground formations. Sonic or acoustic logging tools are accordingly utilized during various phases of hydrocarbon development and exploration.
For example, a sonic logging tool may be lowered by a logging cable into an open borehole. Such a tool, sometimes referred to as a sonde, typically contains one or more sonic wave generators or sources (transmitters), and one or more sonic wave receivers (typically hydrophones), separated by a known distance on the tool. The sonic logging tool emits or xe2x80x9cfiresxe2x80x9d sonic waves, typically in the form of pulses, in accordance with an excitation or drive voltage waveform applied to the transducer of the sonic wave source. These transmitted sonic waves pass through the formation around the borehole and are then detected at the receiver(s). The detected acoustic signals are then typically transmitted to the surface, via a wireline inside the logging cable, for example, for processing, storage, monitoring, or other purposes. In addition to open-hole measurements, a sonic logging tool may also be used to make cased-hole measurements.
Sonic waves can travel through rock formations in essentially two forms: body waves and surface waves. There are two types of body waves that travel in rock: compressional and shear. Compressional waves, or P-waves, are waves of compression and expansion and are created when the rock formation through which the sonic waves travel is sharply compressed. With compressional waves, small particle vibrations occur in the same direction the wave is traveling. Shear waves, or S-waves, are waves of shearing action as would occur when a body is struck from the side. In this case, rock particle motion is perpendicular to the direction of wave propagation.
Surface waves are found in a borehole environment as complicated borehole-guided waves which come from reflections of the source waves reverberating in the borehole. The most common form of borehole-guided surface wave is the Stoneley (St) wave. Such sonic waveforms may be detected by a receiver as a result of sonic waves generated or emitted from a monopole (omnidirectional, or symmetric) source, for example. A monopole source generates primarily an axisymmetric family of modes together with compressional and shear headwaves.
Dipole (directional sources and receivers may also be used in some applications. A dipole source excites the flexural family of borehole modes together with compressional and shear headwaves. The flexural mode waves may be referred to as flexural waves. Sonic waves will also travel through the fluid in the borehole and along the tool itself. With no interaction with the formation, these waves carry no useful information and can interfere with the waveforms of interest if they have similar propagation speeds.
A dipole transmitter may consist essentially of a moving coil loudspeaker capable of radiating pressure pulses from both sides of its xe2x80x9cspeaker-cone.xe2x80x9d The speaker-cone is typically a piezoelectric source or disk such as a 2xe2x80x3 diameter titanium disk. Thus, when a current pulse (having a drive or excitation waveform) passes through the coil, the disk vibrates parallel to its axis, creating positive pressure on the borehole fluid on one side of the sonde, and a negative pressure on the other side. Thus, when dipole sources are employed, an additional shear/flexural wave propagates along the borehole and is caused by the flexing action of the borehole in response to the dipole signal from the source. The receivers may be hydrophone receivers, placed on the tool along the borehole axis a known distance from each other and from the sonic generator(s).
Various types of dipole signal sources and transmitters have been employed or proposed. These include, for example, electromagnetic transducer devices such as is used in Schlumberger""s DSI tool (see U.S. Pat. No. 4,862,991 (Hoyle et al.), issued Sep. 5, 1989; U.S. Pat. No. 4,207,961 (Kitsunezaki), issued Jun. 17, 1980; U.S. Pat. No. 4,383,591 (Ogura), issued May 17, 1983); linked mass vibrators driven by magnetostrictive actuators (see, e.g., S. M. Cohick and J. L. Butler, xe2x80x9cRare-Earth Iron xe2x80x98Square Ringxe2x80x99 Dipole Transducer,xe2x80x9d J Acoust. Soc. Am. 72(2) (August 1982), pp. 313-315); piezo-electric bender devices such as are used in the XMAC tool of Baker Atlas (see, e.g., U.S. Pat. No. 4,649,525 (Angona et al.), issued Mar. 10, 1987); magnetic repulsion transducers driving a plate in contact with a fluid in an acoustic wave guide system such as are used in the MPI XACT tool (see, e.g., U.S. Pat. No. 5,852,262 (Gill et al.)); and eccentric orbital masses as proposed in U.S. Pat. No. 4,709,362 (Cole) and U.S. Pat. No. 5,135,072 (Meynier), mainly for seismic uses.
The speeds at which sonic waves travel through underground rock formations are controlled by rock mechanical properties such as density and elastic dynamic constants, and other formation properties such as amount and type of fluid present in the rock, the makeup of the rock grains, and the degree of intergrain cementation. Thus, by measuring the speed of sonic wave propagation in a borehole, it is possible to characterize the mechanical attributes of surrounding formations by parameters relating these properties, which are necessary in efficient and safe development of oil and gas wells. The speed or velocity of a sonic wave is often expressed in terms of I/velocity. Since the acoustic sources and receivers on the tools are separated by a known, fixed length, the difference in time (xcex94T) taken for a sonic wave to travel between two points on the tool is directly related to the speed/slowness of the wave in the formation. Thus, surface processing equipment typically determines the speed of a given type of sonic wave by determining the measured travel time between the source and receiver, for various types of acoustic waves. The speed of a given type of wave emitted at a given frequency may then be used to determine formation properties, through suitable processing.
For example, acoustic measurements may be used for: identification of homogenous versus fractured rocks; estimation of rock porosity; identifying oil- versus gas-filled porous formations; identification of near-wellbore invasion of mud fluid in a porous formation; over-pressured regions of the formation; and the presence of large tectonic stresses that can produce radial alteration in the borehole vicinity. Such sonic logging techniques are well known. See, for example, Jay Tittman, Geophysical Well Logging, Orlando, Fla.: Academic Press (1986); Illustrated Physical Exploration, Physical Exploration Society (1989); Bikash K. Sinha and Smaine Zeroug, xe2x80x9cGeophysical Prospecting Using Sonics and Ultrasonics,xe2x80x9d in John G. Webster, ed., Wiley Encyclopedia of Electrical and Electronics Engineering, New York: John Wiley and Sons, Inc. (1999), pp. 340-365.
Dipole sonic logging tools are sometimes preferred to monopole sonic logging tools, because the former cannot acquire shear xcex94T in formations where the shear slowness is in excess of the fluid compressional slowness (180-200 xcexcs/ft). Using dipole sonic propagation in the borehole, this physics limitation is removed and shear slowness far in excess of the fluid slowness may be measured. As noted above, a dipole transmitter behaves much like a piston, creating a pressure increase on one side of the borehole and a decrease on the other. This, in turn, causes a small flexing of the borehole, that directly excites compressional and shear waves in the formation. The compressional wave radiates most strongly straight out through the formation, but the shear waves tend to propagate best along the borehole walls. As the shear wave propagates up the borehole, it creates a pressure difference in the borehole fluid that propagates along with it. It is this pressure difference that is detected by the sonic logging tool""s directional sonic receivers. Unlike monopole sonic tools, dipole tools can always record a shear wave, regardless of wave speed.
Tagging along behind the dipole shear wave is a flexural wave, which is initiated by the flexing action of the borehole. Flexural waves are typically very dispersive (i.e., their velocity is a function of frequency). Flexural waves are relatively long in duation, since its low-frequency components propagate with the formation shear slowness, i.e. travel at the same speed as the shear slowness waves (but slower than compressional waves). Thus, even though the formation shear and flexural waves run together, the shear can often be detected and, as a result, the formation shear slowness can be determined or estimated directly from both flexural dispersion and actual shear wave measurements. For example, radial variation of shear velocity can be estimated based on measured borehole flexural dispersion. See, for example, U.S. Pat. No. 5,587,966 (Kimball et al.), issued Dec. 24, 1996, describing techniques for deriving borehole information from measured flexural waves.
Historically, dipole logging employing flexural wave measurement has been developed as an alternative to shear wave logging, to be employed in cases where the shear slowness of the formation was so slow that shear head waves could not be generated in the borehole. Flexural wave measurement is based on the fact that the slowness of the flexural wave approaches the shear slowness of the formation.
However, flexural waves are believed to carry richer information than shear waves do. The behavior of flexural waves is sensitive to various mechanical properties of the boreholes. In particular, the dispersion relation of flexural waves implies various useful information such as stress-induced anisotropy. The shear slowness of the formation can be estimated from dispersion curves of measured flexural waves, especially in xe2x80x9cslowxe2x80x9d formations.
Further, as knowledge of flexural mode propagation increases, flexural logging may be extended to determine other useful information. For example, it may also be used in anisotropic processing, in which a tool, such as a DSI tool, is run in fast formations in order to find anisotropy in the radial plane with respect to the borehole. Such anisotropic processing is used to determine properties of the formation around the borehole which are different depending on the azimuthal direction. For acoustic logging, anisotropy generally appears as a difference in propagation speeds in orthogonal radial planes. Typically, there is a slow-direction plane and a fast-direction plane at 90 degrees to this. This anisotropy can be related to a number of causes, for example, stress in the formation. Such information can be useful, therefore, for example, in designing a fracturing operation to stimulate production from the formation.
It can, therefore, be useful to excite, and detect and analyze, flexural waves. Downhole tools with dipole sources typically transmit sonic waves at a single, fixed firing frequency. However, the sonic frequency of the dipole transmitter may not be optimal for a given formation and borehole at a certain position in the borehole, generating less optimal and thus less useful flexural waves than would be possible were an optimal or ideal sonic frequency and range utilized by the transmitter for a given measurement. One reason for this is that the bandwidth needed for dispersion analysis varies as function of borehole diameter and formation slowness. Thus, for a given formation, if a non-optimal firing frequency is used, optimal flexural wave excitation will not occur. Thus, when detecting and analyzing flexural mode in the received sonic waves, less information will be obtainable than if more energy were to be coupled into the flexural mode waves, i.e. if the flexural waves were more optimally excited by the transmitted sonic waves.
To ensure that the appropriate excitation was provided for a given application, one possible approach would be to provide a wide-band dipole source which generates sonic pulses over a wide range of frequencies, to ensure that the optimal sonic wave frequency, whatever it is, is transmitted. However, this approach would requires an acquisition system having a much larger (and thus more expensive and complicated) dynamic range, and a much higher consumption of power in the generation of such a wide band of signals. Because real-world logging tools have transmitters with limited power and receivers with finite dynamic range, it is often impossible to transmit a sufficiently powerful, wideband signal, and thus a narrower frequency range is employed. In this case, however, if the wrong firing pulse frequency is used for given borehole and formation characteristics, it may not be possible to obtain a useful signal over enough frequency band. Another problem with using a wide-band dipole source is that the resulting sonic waveforms may contain unwanted modes such as dipole compressional waveforms.
Thus, as the potential use of dipole sonic logging increases, there is a need for selecting an optimal sonic dipole firing frequency (source signature) to optimally excite dipole flexural mode waves in the borehole.
In the present invention, a logging system has a sonde in a borehole, which transmits sonic waves with one or more dipole sources to optimally excite dipole flexural mode sonic waves in the borehole. The system first determines the Airy frequency of the borehole. Then, the dipole sources are fired at a dipole firing frequency selected based on the Airy frequency.